Abstract:

Disturbance, whether natural or of human origin, modifies to varying degrees
many ecosystem attributes. Fire is a natural process in the montane forests of southern
Oregon but for much of the 20th century fire was viewed as an apocalypse and thus
fervently suppressed. Effective natural resource management requires an
understanding of how ecosystems function, including the ecological response to the
natural disturbance regime or anthropogenically altered conditions.
To describe the effects of natural fire and ecosystem changes with time
following fire, I examined plant composition, structure, biomass and soil nitrogen
using a chronosequence of sites ranging in age from 1 to >300 years. The ecological
importance of human activity is a function of the magnitude and permanence of its
effects which were described at active and abandoned recreation sites.
Stand-replacing fires killed virtually 100% of the overstory trees, consumed
87% of the forest floor and resulted in a 27% loss in total aboveground biomass. A
pulse of mineralized nitrogen was still apparent 2 years following fire.
Two years following fire, little plant colonization was established. The
availability of local seed sources, particularly seedbanking species, appears to have
been an important determinant of the early-successional community composition. The
developing community was dominated by nitrogen-fixing species which may play an
important role in the post-fire nitrogen dynamics in these ecosystems. Conifer
seedling densities measured 667 individuals/ha 8 years following fire and 1714
individuals/ha 24 years after fire.
Extended tree maturation and variation in composition characterized the forest
structure and composition during secondary succession. Total aboveground biomass
remained high and accumulation followed a general U-shaped curve.
Recreational use resulted in a reduction in tree density and biomass at all active
sites. The greatest reductions occurred at sites set in late-successional forests.
Compositional change was also greatest at sites established in late-successional forest
with increased dominance of lodgepole pine and graminoids. Within 40 years the
abandoned, mid-successional sites had recovered from most of the recreational
impacts; however 40 years was not sufficient for late-successional forest recovery.
These results suggest that the stage of succession should be considered when assessing
potential recreational impacts.